1. Field of the Invention
The present disclosure generally relates to compositions, fluids, and methods for treating subterranean formations. More particularly, the present disclosure relates to drilling and servicing fluids comprising particulate agents, which may also be biocide precursors, and methods for treating subterranean formations using such fluids.
2. Description of the Related Art
In the oil and gas, water well, and injection well industries, boreholes are drilled into subterranean formations and certain fluids may then be introduced therein. The fluids can flow through vugs, pore spaces, and natural or manmade fractures in the formations. Additional flow paths in the formations may also exist. For example, fluid can flow through the wellbore, a well casing, perforation tunnels, slots, tubing, annuli, washouts, particulate packs, screens and completion, service, or workover equipment.
During the producing (or injecting) phase of a well, it is generally desirable to have the subterranean formation as free flowing as possible or at a rate set by the design of the well. However, during the drilling, completion, or workover phases, it may be desirable to allow various fluids to flow into only a portion of the formation or wellbore while preventing it from flowing into other portions. Particulate material has been employed to bridge/seal/plug certain flow paths in the formations where the flow of fluids is undesirable. The different materials that have been used historically for these purposes can generally be classified as permanent, removable, or temporary.
Examples of commonly used permanent materials are sand, clay, barite, and a number of different waste stream materials, such as fly ash. Although these materials function to control fluid flow/loss at the time of use, they have a severe residual effect on the production or injection of the well later in time. An example of a removable material is calcium carbonate. After the calcium carbonate has been used to modify flow, a cleanup solution, such as an acid, will need to be injected to contact the calcium carbonate and remove it from the flow paths. Examples of temporary materials are those that degrade, decompose, or have gradual solubility in the wellbore fluid. Biodegradable polymers, such as polylactic acid, are commonly used since they degrade in the presence of small amounts of water. However, these polymers have temperature and cost limitations that prohibit their use in many wellbore applications. Further, decomposition of these materials does not lead to the formation of any particularly useful products, such as biocides.
During the drilling and completing of oil and gas wells, precautions must be taken to prevent the introduction of significant numbers of sulfate reducing bacteria (SRB) into the zone. These bacteria can grow in population in fluids that contain food sources for them, generally sulfate sources (ex: lignosulfonates, gypsum) and certain organic materials (ex: polysaccharides). The bacteria can be easily picked up in seawater and in water from retaining ponds. Placement of a fluid containing SRB's downhole can generate two conditions favorable to population growth. The first is a reduced oxygen environment (anaerobic). SRB's are common anaerobic bacteria. The second is the additional sources of sulfate and sulfite that can be found in some formations. Under these conditions the SRB's convert the sulfate (or sulfide) into sulfide, generally as the gas “hydrogen sulfide.” Oil or natural gas that contains hydrogen sulfide (H2S) gas is referred to as “sour.” Because of the additional cost of refining hydrocarbons containing H2S, the crude oil/natural gas from these reservoirs have a reduced value. In addition, the presence of these bacteria and their byproducts can reduce the production capability of a field.
A third issue is the reaction of H2S with steel, causing corrosion that can lead to significant mechanical problems with well and production equipment. Given how easy it is to have these bacteria contaminate oil field fluids and the commercial damage they can cause, additives to prevent the bacteria population from growing can be highly desirable.